The correct answer requires two firewall policy configuration elements: a deny policy blocking all traffic from the Guest VLAN to the internal network, and an explicit accept policy for DNS traffic placed above it. This works because FortiGate firewall policies are evaluated sequentially from top to bottom, so the specific DNS accept rule must precede the broader deny rule to create an exception—otherwise, the deny-all rule would match and drop the DNS packets first. On the Fortinet NSE 4 Network Security Professional NSE4 exam, this scenario tests your understanding of policy ordering and the implicit deny principle, often appearing as a trick where candidates forget that an explicit allow for the exception must be placed before the blanket block. A common trap is selecting only the deny policy, missing the critical order-dependent accept rule. Memory tip: think "Allow the exception before the rejection"—like a bouncer letting in VIPs (DNS) before locking the door to everyone else.
NSE4 Firewall Policies and NAT Practice Question
This NSE4 practice question tests your understanding of firewall policies and nat. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A FortiGate admin needs to block all traffic from the 'Guest' VLAN (192.168.100.0/24) to the internal network (10.0.0.0/8) except for DNS traffic (UDP 53) to the internal DNS server at 10.0.0.10. Which TWO firewall policy configuration elements are required to achieve this? (Choose two.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A firewall policy with source 'Guest' VLAN, destination 'Internal network', service 'ALL', action 'deny'
Option B is correct because a deny policy with source 'Guest' VLAN (192.168.100.0/24), destination 'Internal network' (10.0.0.0/8), and service 'ALL' will block all traffic from the Guest VLAN to the internal network. Option C is correct because an explicit accept policy for DNS (UDP 53) to the internal DNS server (10.0.0.10) must be placed before the deny policy, as FortiGate firewall policies are evaluated in order from top to bottom, and the first matching policy determines the action.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
An address group for the internal DNS server
Why it's wrong here
An address group is not strictly required; a single address object suffices.
✓
A firewall policy with source 'Guest' VLAN, destination 'Internal network', service 'ALL', action 'deny'
Why this is correct
This policy will block all other traffic from Guest to internal.
Related concept
Read the scenario before looking for a memorised answer.
✓
A firewall policy with source 'Guest' VLAN, destination 'Internal DNS server', service 'DNS', action 'accept'
Why this is correct
This policy will allow DNS traffic to the specific server.
Related concept
Read the scenario before looking for a memorised answer.
✗
A traffic shaper to limit DNS traffic
Why it's wrong here
Traffic shaping is not required for access control; it only affects bandwidth.
✗
A schedule object to apply the policies only during business hours
Why it's wrong here
The requirement does not mention time restrictions.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often think an address group (Option A) is necessary for the DNS server, but a single address object works just as well, and the real key is the policy ordering between the explicit accept and the explicit deny.
Detailed technical explanation
How to think about this question
FortiGate firewall policies use a first-match model, so the explicit DNS accept policy must be placed above the deny-all policy to ensure DNS traffic is permitted before the broader deny rule is evaluated. The source and destination addresses in the policies can be defined using address objects or directly as subnets; the deny policy should use the 'Internal network' object covering 10.0.0.0/8, while the accept policy uses a specific address object for 10.0.0.10. In real-world deployments, administrators often forget to order policies correctly, leading to unintended blocking of DNS resolution for Guest users.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Firewall Policies and NAT — This question tests Firewall Policies and NAT — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: A firewall policy with source 'Guest' VLAN, destination 'Internal network', service 'ALL', action 'deny' — Option B is correct because a deny policy with source 'Guest' VLAN (192.168.100.0/24), destination 'Internal network' (10.0.0.0/8), and service 'ALL' will block all traffic from the Guest VLAN to the internal network. Option C is correct because an explicit accept policy for DNS (UDP 53) to the internal DNS server (10.0.0.10) must be placed before the deny policy, as FortiGate firewall policies are evaluated in order from top to bottom, and the first matching policy determines the action.
What should I do if I get this NSE4 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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